1. Field of the Invention
The present invention relates to an optical fiber cord with a small diameter suitable for high-density assembly of wirings between central office equipments, central office optical wirings, or the like, a ribbon cord using the same, and a ribbon cord branch line using the same.
2. Description of the Related Art
The request for higher density assembly of the central office equipments has been increased according to the spread of optical fiber construction in the subscriber's system. In order to satisfy such a request, an optical fiber cord with a small diameter, which is to be used as wirings in or between central office equipments, etc., has been needed.
The optical fiber cord with a small diameter according to a first related art is constructed such that a yarn made of aramid fiber, glass fiber, carbon fiber, etc. are provided longitudinally along a UV curable resin coated optical fiber of a diameter of 250 .mu.m and then ultraviolet curing resin is coated thereon and cured. A fiber-reinforced covering layer which is formed by distributing reinforcing fibers integrally into the ultraviolet curing resin is then provided. In general, the optical fiber cord having the above configuration has a finished diameter of about 0.5 to 1 mm.
However, in the optical fiber cord according to the first related art, the reinforcing fiber serving as a tension member has been formed like a yarn. Since the yarn has not been tight, a packing factor (ratio of true volume to bulk volume) of the reinforcing fiber in the fiber-reinforced covering layer has not been able to be increased more than 50 volumetric %, so that it has been impossible to be accomplished sufficiently high reinforcement per unit cross section of covering layer. Accordingly, it has been difficult to be accomplished sufficiently high reinforcement in case of making covering layer be smaller in thickness. For example, in the prior art, it has been impossible to fabricate the optical fiber cord with an extremely small diameter of about 250 .mu.m as the finished diameter.
Hence, it may be considered to increase a packing factor of the fiber-reinforced covering layer by applying monofilament aramid fibers to the optical fiber cord according to the first related art. However, it has been difficult to form such aramid fiber as a monofilament. Since the result ant monofilament is thus inferior to the yarn in characteristic, the sufficiently high reinforcement effect has not been able to be expected.
Otherwise, it is sufficiently possible to employ monofilament glass fibers in the optical fiber cord according to the first related art. However, since such monofilament lacks flexibility, the monofilament has not been satisfied in reliability due to the breakdown during operation.
FIG. 1 is a schematic lateral sectional view showing an optical fiber cord with a small diameter according to a second related art.
The optical fiber cord according to the second related art is constructed such that a yarn 103 made of aramid fiber, glass fiber, etc. are provided longitudinally as a tension member around a UV curable resin coated optical fiber 102 and then a covering layer 104 made of ultraviolet curing resin, etc. is coated thereon and integrally cured.
However, in the optical fiber cord according to the second related art, the reinforcing fiber serving as tension member has been formed like a yarn. Therefore, a rate of the yarn 103 occupied in the covering layer 104 has not been able to be increased more than 50 volumetric %. For this reason, since it has been impossible to be accomplished high reinforcement per unit cross section of covering layer in case of making the covering layer e smaller in thickness, it has been difficult to reduce the finished outer diameter less than 0.5 mm.
In addition, a ribbon cord which is constructed by arranging a plurality of the optical fiber cords in parallel and forming integrally by a matrix material has not been made small in thickness and width similarly, so that such ribbon cord has not satisfied the request for higher density assembly of the central office optical wirings.
Moreover, in the ribbon cord employing the optical fiber cord according to the second related art, the yarn 103 serving as the tension member is provided around the UV curable resin coated optical fiber 102. Because of the presence of the yarn 103 provided between adjacent UV curable resin coated optical fibers 102, it has been difficult to expose top ends of the UV curable resin coated optical fibers 102 by removing the matrix material and the covering layer 104 together. There has been such a disadvantage that, since a dimensional discrepancy because of the intervention of the yarn 103 resides between the ribbon cord employing the optical fiber cord according to the second related art and the ordinary optical fiber ribbon, it is impossible to collectively splice them.